iterators: several small fixes (#11162)

* fix pairs, mpairs, mitems for cstring for JS backend
* add mutation check for pairs and mpairs for strings and seqs
* change the assertion message - modifying (changing elements) is not
  a problem, changing the length is

tests/ccgbugs/twrong_tupleconv.nim

@@ -6,7 +6,7 @@ iterator myitems*[T](a: var seq[T]): var T {.inline.} =
   while i < L:
     yield a[i]
     inc(i)
-    assert(len(a) == L, "seq modified while iterating over it")
+    assert(len(a) == L, "the length of the seq changed while iterating over it")
 
 # Works fine
 var xs = @[1,2,3]